I don't know what source you were reading, or when you were a teenager, or whether recommendations are different in different countries.
I've lived in the USA my whole life, and I've seen many sources, over quite a few decades, that consistently recommended 4 square feet per chicken (sometimes broken down as 4/3/2 square feet for large/small/bantam breeds.)
For more nuanced guidelines, I can offer quotes from two chicken books with very different publication dates, but neither is exactly new. One is meant for backyard flocks, the other for commercial farmers.
From 1976: "If you plan to raise large breeds at least 4 square feet will be needed for each bird, and bantams should have not less than two square feet each. Of couse, if the chickens will be confined to the coop for a great part of the time, they will need more room; allow ten square feet for each large bird..."
(Chickens in Your Backyard, by Rick & Gail Luttmann)
From 1925:
"The smaller the flock, the more floor space is required per hen. Ten hens might need 6 to 8 square feet per bird; while 125 hens might be kept profitabley with an allowance of 3.2 sq. ft. per hen."
(Practical Poultry Management, by James E. Rice and Harold E. Botsford)
(I can't check most of the other books I used to read, because they are not new enough to be in my local library, but not old enough to be out-of-copyright and posted online. But I clearly remember that 4 square feet per hen was always the default for the ones that didn't bother to give more nuanced guidelines.)